A variety of technologies exist to determine the position of a stylus, or even a finger, placed on a surface. One technology is a grid of horizontal and vertical wires that are placed below the surface of a flat tablet or over the surface of a display device and emit position indicating signals which are detected by a stylus. Two devices using this type of technology are described in U.S. Pat. Nos. 5,149,919 and 4,686,332 to Greenias, et al. Applications using these devices are computer input drawing (or digitizing) tablets, and touch-screen display devices.
In another technology, surface acoustic waves are measured at the edges of a glass plate and are used to calculate the position on the plate that was selected by a finger or a stylus. Applications include high use touch screen kiosk displays where a conductive overlay technology would wear out.
Yet other technologies include the use of light pens as optical detectors. Additionally a frame around a flat display with an array of light emitters and detectors around the edge of the frame, may be used to detect when a finger or stylus is near the display surface. These technologies are limited to displays or flat surfaces.
Position detectors such as the devices disclosed in the Greanias patents, that use many conductors arranged in a grid, are not well suited to a complex shaped surface of either two or three dimensions. There are, at a minimum, difficulties in positioning and shaping the conductors to fit the contours of a complex shape.
Another similar device is a grid of horizontal and vertical wires placed over or beneath the surface of a flat display device that uses capacitive coupling of a stylus or finger. In this device, the capacitive coupling transfers position indicating signals from one wire to another which can be used to calculate the position of the coupling. Computer input tablets, as well as finger pointing mouse replacement tablets, use this technology.
In another technology, a rectangular homogenous transparent conductor is placed over the surface of a display device and bar contacts on the edges of the transparent conductor charge the conductor. Capacitive coupling of a stylus or a finger to the transparent conductor causes the conductor to discharge while sensors attached to the bar contacts measure the amount of current drawn through each of the contacts. Analysis of the ratios of the currents drawn from pairs of contacts on opposing sides of the rectangle provide an X-Y position on the panel that was selected by the user. A device of this type is described in U.S. Pat. No. 4,853,498 to Meadows, et al. An application of this device is a touch-screen display.
A similar technology uses a rectangular piece of extremely uniform resistive material with a series of discrete resistors along the edge and is mounted on a flat surface. A voltage differential is applied to the row of resistors on opposing sides of the rectangle and in a time-division manner the voltage differential is applied to the row of resistors of the other two opposing sides. The position indicating signals are either received by a stylus, or by a conductive overlay which can be depressed to contact the surface of the resistive material. One variety of this device is described in U.S. Pat. No. 3,798,370 to Hurst.
The devices described in U.S. Pat. Nos. 4,853,498 (Meadows, et al.) and 3,798,370 (Hurst) drive a homogenous rectangular resistive overlay with bar contacts or a string of resistors along each edge. These approaches rely upon the regular shape of a rectangle in order to work. The shape and placement of the contacts provide the means to detect portions of the surface within a rectangular subsection of the resistive material of the surface. Other simple shapes may also be feasible with bar and resistor string contacts but in complex shapes they can create areas that cannot be distinguished (e.g., shapes with concave edges such as a circle or ellipse can not be accommodated by either the Meadows or the Hurst approaches). The use of bar contacts or strings of resistors along substantially the entire edge of an object limits their usefulness on objects where the position on the entire surface needs to be detected. The locations directly beneath each bar electrode and between each bar or spot electrode and the edge of the object are not detectable in these devices.
The devices described in U.S. Pat. Nos. 4,853,499 (Meadows, et al.) and 3,798,370 (Hurst) do not take into consideration the effects of contact resistance. The resistance between the contacts and the homogenous resistive material may be substantial relative to the resistance of the homogenous material. Additionally the contact resistance may vary from electrode to electrode or change due to mechanical or environmental stress. The Meadows and Hurst devices rely on contacts of known, or constant resistance, which constrains the use of materials and contact approaches. Any variation in contact resistance or changes in contact resistance due to environmental factors are not accounted for and result in detection errors.
Further, Meadows loads the surface with a capacitively coupled stylus and determines position by measuring the current drawn from the driving circuits. The Meadows device requires four receiver circuits to accomplish this.
The Meadows device is susceptible to the effects of unwanted phantom styluses coupling to the surface. Phantom styluses such as rings or fingers may couple to the active surface instead of, or in addition to, the actual stylus. These phantom styluses cause detection errors because the changes that they also produce cause changes in the driving circuit.
In applications where the object containing the grid needs to be rotated, or the electronics and the object are physically spaced-apart from each other, a large number of conductors must be coupled to the system, or between the elements of the systems, through connection mechanisms that may allow rotation or other movements. Such cables for the systems of the prior art would be rather large and cumbersome. Further, connectors with a large number of contacts are expensive and reduce the overall reliability of any system that requires them. Contacts that allow rotation, such as slip rings or commutators, become prohibitively complex and expensive as the number of connections rises above a small number. Additionally, the multiple circuits required to drive grid arrays are complex and costly to manufacture. Acoustic wave detectors provide a rugged position detection mechanism but are costly to implement. Light wave detection mechanisms are limited to flat surfaces and are susceptible to dust and insects blocking the light paths. It is believed, however, that the present invention solves these problems.
In today's modern environment there are many sources of electro-magnetic energy, both naturally occurring and man-made. Some examples of the sources of such energy in the earth's atmosphere are static electricity, electrical storms, heat lightning, radiation from outer space, and man-made radio waves. Each of these acts and interacts with each other causing interference and background noise to each other, depending on the intensity of the background or interfering signal. Thus, as is well known in devices that utilize an antenna as a device to detect an input signal, these atmospheric signals may interfere with the ability to detect and receive a signal of interest. It is also known that in systems with a hand-held antenna probe, the human body acts as a larger antenna with a signal from the person holding that probe added to the signal of interest detected by the hand-held probe. That added signal, and the multiple frequencies that it includes is also known to potentially add a level of inaccuracy in such a system, if the desired signal can be detected at all. To overcome that unwanted interference many elaborate circuits have been devised to suppress those interference signals "picked-up" by the human user from impacting the performance of the system.